Lanthanum (La)

The ground-state configuration of lanthanum is [Xe] 5d¹ 6s² (often written with an empty 4f subshell: 4f⁰). Because La precedes the filling of the 4f orbitals across the series (Ce–Lu), it is commonly treated as the gateway to the lanthanides and is frequently grouped with them in chemistry and materials science.

Lanthanum overwhelmingly forms the +3 oxidation state (La³⁺). Typical compounds include:

• Lanthanum(III) oxide, La₂O₃ (basic oxide)
• Lanthanum(III) chloride, LaCl₃ (water-soluble salt)
• Lanthanum(III) hydroxide, La(OH)₃ (sparingly soluble, gelatinous precipitate)

Lanthanum tarnishes in air and forms the oxide; it also reacts slowly with water to liberate hydrogen gas:

Freshly cut surfaces should be handled under oil or inert gas to limit oxidation.

Lanthanum(III) is 4f⁰ with no f–f electronic transitions. Many transition-metal colors arise from d–d transitions; in La³⁺ these are absent, so aqua La³⁺ solutions are typically colorless.

The lanthanide contraction is the steady decrease in ionic radii from La³⁺ to Lu³⁺ due to poor shielding by 4f electrons. La, at the start with 4f⁰, has the largest Ln³⁺ radius; moving across the series increases effective nuclear charge felt by valence electrons, contracting the ions and subtly affecting periodic trends of following elements (e.g., Hf vs. Zr).

Lanthanum is found in bastnäsite and monazite minerals along with other rare-earth elements. Industrial separation uses solvent extraction and ion exchange exploiting small differences in complex stability and ionic radius among Ln³⁺ ions.

Key applications include:

• Optical glass (La₂O₃ increases refractive index and lowers dispersion for camera and telescope lenses).
• FCC catalysts (La-modified zeolites in petroleum fluid catalytic cracking).
• Rechargeable batteries (hydrogen-storage alloys such as LaNi₅ in Ni–MH cells).
• Perovskites (LaMnO₃, LaCoO₃) for catalysis, electrodes, and electronics.

In water, La³⁺ forms highly hydrated complexes like [La(H₂O)₉]³⁺ and undergoes hydrolysis to yield La(OH)₃ at higher pH:

The hydroxide is a gelatinous, weakly basic precipitate that can redissolve in strong acids.

Alloys such as LaNi₅ reversibly absorb hydrogen to form hydrides (e.g., LaNi₅H_x), enabling Ni–MH batteries. Hydrogen cycles between metal hydride (anode) and nickel oxyhydroxide (cathode):

La contributes to favorable plateau pressures and cycling stability.

Metallic La is considered of low acute toxicity, but fine powders are reactive and can be irritants; some soluble salts may affect mucous membranes. Avoid inhalation/ingestion, use gloves and eye protection, and handle powders under ventilation. Store metal under oil or inert gas to prevent oxidation and moisture reactions.

Formation of the oxide and hydroxide, and a simple salt metathesis: